1. Field of the Invention
The invention relates generally to magnetic memory storage devices for computers and more specifically to a method of writing embedded servo tracks and/or servo-patterns onto magnetic surfaces of disk drives using the actuator and magnetic read/write transducer of the drive.
2. Description of the Prior Art
Increasing data storage capacity of magnetic memory disk storage devices such as floppy and hard disk drives requires higher track densities or pitch. Such devices typically utilize voice-coils and other types of precision servo-responsive positioning mechanisms for locating a magnetic read/write transducer incorporated into an air bearing slider `flying` above a spinning disk surface (a slider head) gimbaled at the end of a suspension arm. The slider read/write head both `writes` magnetic data signals into a magnetically susceptible material of or coating the disk surface and `reads` magnetic data signals previously recorded/stored in the material or coating. Because the data signals written to and read from a disk surface typically comprise a stream of digital pulses, a mechanism must be provided for the servo positioning mechanisms to determine both radial and angular (circumferential) positions of the slider read/write head relative to the surface of the spinning disk. High storage capacity magnetic memory disk storage devices require magnetically embedded servo tracks pre-recorded on the disk surface to provide positioning signals via the read/write head enabling the precision servo-responsive positioning mechanism to determine and adjust the location of the slider head above the spinning disk surface.
Existing manufacturing techniques for high storage capacity disks normally utilize independent precision servowriter machines for writing the embedded servo tracks on the magnetically susceptible material of or coating on the disk of a head/disk assembly (HDA). Such machines are generally one-of-a-kind precision devices which typically rely on laser driven optical and other feedback mechanisms for establish physical position of a recording head used to write embedded servo tracks. Unfortunately, the combined hysteresis, hysteretic and other error inherent both in independent servo track writing machines and in servo-responsive positioning mechanisms of the HDA preclude track pitches much below 200 .mu.inches (5000 tracks/inch). Moreover, it is impossible to position a dynamic mechanical system such as an HDA within another dynamic mechanical system of an independent servowriter machine without producing mis-positioning error. This is because the servowriter machine is writing coordinate data for the HDA relative to a coordinate system inherent in its dynamic optical, electrical and mechanical components, not the coordinate system inherent in the dynamic electrical and mechanical components of the HDA.
For an explanation of the problems created by mis-positioning errors, and the steps that must be taken to correct for them, reference should be made to US. Pat. No. 4,536,809 issued May 10, 1982, entitled Adaptive Misposition Correcting Method and Apparatus for Magnetic Disk Servo System, by Michael Sidman. In particular, Sidman teaches a method for improving the track following capability of the servo-responsive positioning mechanisms locating and maintaining a slider head of an HDA above a track. More precisely, Sidman describes a method for providing corrective electrical signals to servo-responsive mechanisms for adjusting the radial position of the slider head with respect to the disk spin axes for the purpose of maintaining the slider head over the centerline of a track which is not radially concentric with the disk spin axes. According to Sidman the centerline of a track is defined or established by a pre-recorded embedded servo track most probably written on the disk surface by an independent servo writing machine. One of the typical mis-positioning errors described by Sidman is a mechanical disturbance termed spindle `wobble` or `runout` due to a difference between an actual track centerline and that effectively seen by the slider head at a fixed distance from the spin axis of the disk surface. Sidman, observes that that difference commonly results from a slight eccentricity in mounting of the disk on its drive spindle and gives rise to a position error signal that has a characteristic sinusoidal wave form with a periodicity identical to the rotation velocity of the disk. However, Sidman does not describe, teach or suggest any method, means or mechanism for eliminating, mitigating, or minimizing such mis-positioning error when writing the embedded servo tracks.
Conventional embedded servo-patterns typically comprise short bursts of a constant frequency signal, termed servo bursts, radially aligned on the disk surface. Typically there are two types of servo bursts, variously termed, each located at a different circumferential position on the disk surface and radially offset one track width relative to the other. [Track width is principally determined by the geometry and dimensions of the flux gap of the slider head but varies as will be explained infra.] The boundary between the radially offset servo bursts define a data track centerline on the surface of the disk, i.e., each data track centerline is radially offset 1/2 track width relative to the two types of servo bursts embedded in the disk surface. [See FIG. 4. Sidman (supra)] Accordingly, before reading or writing data, typically the slider head is roughly positioned radially near the location of a desired track around the disk typically by counting tracks crossed or by making a linear and angular head velocity determination. At that rough position, the two types of embedded servo bursts pre-written onto the disk surface produce two distinctive signals trains. The amplitudes of those signal trains are compared to produce a difference signal indicating radial position error where the polarity of that signal indicates the direction of radial misalignment. The servo positioning mechanism moves the slider head responsive to the difference signal in the direction indicated by signal polarity until the head is at radial position where the respective signal trains from the two types of servo bursts are equal, At that point the slider head is deemed to be aligned above the data track centerline and data is read from or written to the disk surface.
The radially aligned servo bursts typically establish sector header areas in each data track around the disk surface. There are usually multiple sectors in each data track. The radially aligned servo bursts are supposed to allow the servo-responsive positioning mechanisms to locate and cause the slider head to follow a particular data track center line around the disk where that track is not concentric with the spin axes of the disk surface. Such radially aligned servo bursts also allow the servo-responsive positioning mechanisms to compensate for mechanical mis-positioning errors due to such things as spindle wobble, disk slip, bearing runout thermal expansion/contraction and the like as explained in detail by Sidman, supra.
In order to write the two types of radially aligned servo bursts to a disk surface, some sort of a timing reference, to the spindle of the disk is required. Conventionally the independent servowriter machine includes an auxiliary clock head which writes a reference timing pattern onto the disk surface. The clock head is then used to read that timing pattern to provide necessary timing signals for aligning the servo bursts. Other existing methods for providing the necessary timing reference contemplate external position encoder disks and/or spindle coupled optical encoders. It is also possible to establish a timing reference using the slider head of the HDA.
For example, in IBM Technical Disclosure Bulletin Vol. 33, No. 5, October 1990, R. V. Fowler & N. J. Granger-Brown, describes a phase lock loop (PLL) technique for writing servo tracks into a disk surface of a head/disk assembly using the slider head of the assembly. As described, the PLL technique requires a single clock track written at the outer diameter of the data recording surface which is divided into two alternate phases termed A and B respectively. The slider head is then stepped inwardly in a half track increments using each phase alternately as a source of clock (timing) information for writing both servo bursts in the sector headers and further clock signals in the other phase in the data regions following the sector headers. According to Fowler & Granger-Brown, stepping the slider head inward in half track increments assures that the clock information written in the previously written track can be read by the slider head, i.e., bit-to-bit synchronism is maintained between tracks. Phase error due to write/read delay track to track is supposed to be compensated for systematically. The result of the PLL, technique developed by Fowler & Granger-Brown is a radial aligned embedded pattern of the alternate A & B phase signals in the data regions of the disk separated by radially aligned servo bursts which establish sector headers on the surface of the disk.
Another technique for establishing a timing reference on the disk surface is to write a clock track using the HDA slider head again near the outer periphery of the data recording region on the surface of the disk, and then to omit (erase) an integral number of contiguous clock transitions creating a `once around reference gap` or index. This `once reference gap` or index can then be propagated inward to subsequent tracks by moving the slider head inwardly in half track increments and writing alternating tracks of A-type and B-type signal bursts synchronous with the clock transitions in the clock track using phase/frequency locking looping techniques. [See Applicant's co-pending application Ser. No 08/274,676 for a more complete explanation of this technique.] Sector information is then typically generated from the `once around reference gap` or index.
While in theory the PLL technique developed by Fowler & Granger-Brown should result in good embedded alternate phase servo bursts written at different circumferential positions within each sector header radially offset a half track relative to each other, there is no assurance that a particular track centerline around the disk established by the boundaries between the two different servo bursts in the sector headers for that track is concentric with either of the adjacent tracks or even the spin axes of the disk.
Fowler & Granger-Brown also fail to explain how the slider head is moved inwardly in half track increments. A possible source of such position data is a change in magnitude of the signal read by the slider head reading as it moves inwardly. For, example in U.S. Pat. No. 4,912,576 issued Nov. 8, 1983 entitled Method for Writing Servo, D. W. Janz, explains that if a flux gap of a slider head sweeps forty percent of a signal pattern previously written to the disk surface, then the read voltage generated by that flux gap should be forty percent of the voltage maximum obtainable from the flux gap when the slider head is aligned dead-center over that signal pattern. In fact, in U.S. Pat. No. 4,912,576, Janz describes a method utilizing the magnitude of the voltage signal from a slider heading reading a track for alternatively writing servo bursts patterns into tracks on one side of a disk for servo and data signal patterns into tracks opposite side of the disk. The apparatus described by Janz has an HDA which includes two slider heads, a data head and a servo head, sharing a common actuator mechanism which read from and write to opposite sides of a spinning disk. After erasure of the disk for initialization, a track of first phase servo burst pattern is written on the servo side of the disk at an outer limit by the servo head. The slider heads are then moved in radially one half of a track, as indicated by the amplitude of the voltage signal produced by the servo head reading the track containing the first phase servo burst pattern, and a first data-track is recorded on the data side of the disk using the data head. The slider heads are again moved-in radially one half of a track, this time as indicated by the amplitude of the voltage signal produced by the data head reading the first data-track, and a second phase servo burst pattern is recorded on the servo side. The process is repeated with slider heads being moved radially inward one half of a track at a time until the respective recording surfaces of the disk are filled with servo and data tracks, i.e., the slider head reach the inner stop. Three different phase servo burst patterns are written into the tracks on the servo side of the disk, one phase servo burst pattern per track with the sequence of the phases of servo burst patterns being repeated every three tracks. According to Janz the three different phase servo burst patterns should be angularly (circumferentially) offset relative to each other. Also, during the servo writing process, per Janz, three different phase data signals are written into the data tracks recorded on the data side of the disk. While Janz indicates that his preferred process of writing servo writes the servo tracks to one surface of a disk surface, it is clear that he contemplates embedded servo containing three different phase servo bursts written on the same side of the disk using the same slider head that writes the data signals. [See U.S. Pat. No. 4,912,576, FIG. 8 and col. 9 11. 26-37].
However, Janz fails to describe, teach or suggest any method, means or other mechanism for eliminating, mitigating, or minimizing mis-positioning errors due to such things as spindle wobble, disk slip, bearing runout thermal expansion/contraction and the like when writing or embedding the servo tracks. Accordingly, even an enbedded servo burst pattern written only to one side of a disk per the methods described by Janz is not likely to assure that a particular track centerline around either side of the disk is concentric with either of its adjacent tracks or even the spin axes of the disk.
In U.S. Pat. No. 4,414,589 issued Nov. 8, 1983, entitled Embedded Servo Track Following System and Method for Writing Servo Tracks, T. H. Oliver et al describe a method for writing embedded servo burst patterns to a disk surface quite similar to those described by Janz and Fowler & Granger-Brown. While the preferred method contemplates multiple heads and disk surfaces, per Oliver et al, with a single slider head and a single disk surface, a reference track is written around the disk by the head and the head is moved inwardly radially, until the signal read by the head from the just written reference track equals an arbitrary percentage less than the track center line signal. At that point, a first type (even) of servo burst pattern is written into sector header regions and a second reference track is written into sector data regions.(Sector information is derived from a previously written indexing track) In a similar fashion, the slider head is again moved in until the signal read by the head from the just written second reference track is an arbitrary percentage less at which point a third reference track is written this time only into the data regions of each sector whereupon the head is again moved in with the head reading the third reference track for writing a second type (odd) of servo burst pattern into sector header regions and a fourth reference track into sector data regions. Using techniques similar to those described in Janz and Fowler & Granger-Brown, the slider head is radially stepped in half track intervals across the recording region of the disk alternatively writing even and odd servo burst patterns to the sector headers for every two reference tracks written. The point of departure of Oliver et al relates to use of a summing node means which produces a zero or null signal when the slider head positioned relative to and reading a portion of reference track, produces a signal a desired percentage less than the center track signal.
Oliver et al, like Janz and Fowler & Granger-Brown did not appreciate the necessity for assuring that a particular track centerline around either side of the disk is concentric with either of its adjacent tracks or with the spin axes of the disk. For example, spurious (noise) signals in the circuitry for the slider head are not compensated for or prevented from reaching the summing node means. Accordingly, writing of odd/even & even/odd pairs of servo burst patterns that are neither concentric nor radially offset one track width with respect to each other are not precluded.
Also, as recognized by Oliver et al, the reading and writing performance of slider heads of HDAs are not uniform radially and that head performance is best at the outer periphery of the data region of the disk and deteriorates as head moves radially inward. However, Oliver et al, do not identify possible operative factors which degrade head performance near the inner peripheral of the data region of the disk. Operative factors which degrade the perceived performance of slider heads of HDAs as they move radially inward include a shorter circumferential track length for the same angular or clock (index) interval, i.e., higher data signal densities on disk, and a decrease in surface velocity of the disk surface and a corresponding decrease in flying height and air bearing stability as a function of radial position.
In addition, for higher data storage densities slider heads must fly at elevations 1 to 2 .mu.inches above the spinning disk surface, i.e., at substantially lower elevations than thought possible a decade ago. In effect, from the perspective of the slider head, it actually skims or polishes the peaks of the disk surface material sweeping below it. At such flying elevations, problems associated with friction and temperature are more sever in the interior data region of the disk surface than at the exterior Also, convection cooling is less in the interior of the disk. And, contrary to the observations of Oliver et al, beck,use of lower flying elevations, the disk tracks written in the interior regions of the disk are typically narrower than those written in its outer periphery. Accordingly, the magnitude of obtainable signal from a signal recorded to the interior of a disk is generally less than that obtainable from a signal recorded in peripheral regions of the disk, i.e. the signal-to-noise ratio decreases with radius.
However, it should be noted that it was non uniform radial performance characteristics of the slider heads that stimulated the innovation proposed by Oliver et al a decade ago, of letting the performance characteristics of the slider heads of a particular HDA determine the location and therefore pitch of tracks on its disk surfaces by using those heads to write the embedded servo burst patterns to each disk surface. For current and contemplated high storage capacity magnetic memory disks, techniques implementing the proposal of Oliver et al are a practical necessity.
Mis-positioning errors due to spurious electrical noise, spindle wobble, disk slip, disk tilt, bearing runout, thermal expansion/contraction, external vibration and the like, when writing embedded servo burst patterns using the slider head(s) of the HDA are further compounded by the fact that track width varies as it is being written by a slider head. There are a myriad of factors which affect track width, among them signal strength, flying height, the relative permeability of the magnetically susceptible coating on or of the disk surface, the relative elevations of the surface, gravity, magnetic bias forces, Coriolis forces, head cable forces and windage. Such factors are often interrelated and in many instances derive from the same sources that give rise to other mis-position error.
For example, when writing signals onto a spinning disk surface, where the reluctance of the magnetic circuit across the flux gap between the pole faces of a slider head is greater than the reluctance of the magnetic circuit through the magnetically susceptible material of the spinning disk surface between the pole faces, track width will increase with increasing flying height above the spinning surface, i.e., the track fringes out with increasing flying height until the reluctance of the magnetic circuit through the magnetically susceptible surface of the spinning disk is comparable to that presented by the flux gap at which point the track width will begin to narrow with increasing flying height. Conversely, making the same assumptions, track width narrows with decreasing flying height to equal that of the flux gap when the flux gap, figuratively speaking, is flying through or on the magnetically susceptible material of the disk.
In contrast, when the head is reading or sensing magnetic signals recorded/written in the disk surface, the magnitude of the generated signal is always inversely related to reluctance of the magnetic circuit which increases with increasing flying height. In short, the presumption of prior practitioners, that the magnitude of signal generated in the slider head sweeping over a pattern of magnetic signals written/recorded in a track is linearly or directly related to the proportion of the track sweeping beneath the flux gap of the head is not necessarily correct unless one assumes the head is flying at approximately the same height as it was when it wrote that signal to the disk surface. This later assumption cannot be made.
In particular, every spinning disk has eccentricity meaning that it is typically not mounted exactly coaxial with the spin axis of the HDA. [In fact, a single dynamic spin axis for an HDA can not even be assumed.] This means that the relative surface velocity of a stationary head flying above such an eccentricity spinning disk at any particular radial (or track) position cyclically varies increasing and decreasing flying height, correspondingly increasing (or decreasing) and decreasing (or increasing) the width of a track being written. And, because bearing mechanisms mechanically constraining the spinning disk have runout, it cannot be assumed that any point on the eccentricity spinning disk will follow the exactly the same path or orbit each disk revolution. But, rather, analogous to a point in a vibrating string which describes an elliptical orbit that precesses around the quiescent string position, a point on a slightly eccentricity spinning disk surface should be assumed move in a corresponding precessing elliptical path or orbit.
Other common sources of cyclically induced variation in head flying height include disk tilt, and disk slip. Variations in surface elevation of the disk in different regions can also induce cyclically variation in head flying height that may be the same for a group of adjacent tracks but which may differ between non-adjacent groups of adjacent tracks. [Variations in surface elevation of the disk can stem from standing and moving wave (drumhead) vibrations, surface finishes, variations in coating thickness and the like.] Complicating such cyclic variations in flying height and corresponding variation in track width is Le Chateliers Principle and The Principle of Least Energy.
Le Chateliers Principle provides that whenever a stress is applied to a system in equilibrium, that equilibrium is displaced so as to reduce the affect of that stress. The Principle of Least Energy provides that when a system is in stable equilibrium, any slight change in its condition or configuration requiring performance of work will put it out of equilibrium, so that, if the system is left to its self, it will return to its former state and in so doing will give up the energy imparted when it was disturbed. Lenz's Law, a particular case of Le Chateliers Principle, provides that in case of a change in a magnetic system, that thing happens which tends to oppose that change. [Lenz's Law is the phenomenon utilized to transduce or convert the respective electrical and magnetic signals into each other.] However, in addition to transducing the signals, in simple terms, application of Le Chateliers Principle and the Least Energy Principle to flying heads and spinning disks means that any increases or decreases in magnitude of the electrical and magnetic signals being written to or read from disk necessarily induce corresponding increases or decreases in flying height of the head (and a variation in track width). Also, such increases and decreases induce eddy current forces which tend to decelerate and accelerate the spinning disk.
Because of the inter-relation of the multitude of factors affecting track width and radial position, it can be said that the boundaries or edges of a track being written to disk by a slider head undulate responsive to some factors while the track centerline meanders responsive to those and other factors. The undulations in the boundaries or edges of a track are not necessarily symmetrical relative to track centerline because of variations in forces [torques] tending to twist the flux gap of and the slider head relative to the plane of the disk surface. Both edge undulations and centerline meanders may include cyclic components. Accordingly, when reading an undulating, meandering track, the servo-responsive positioning mechanisms reading servo bursts moves the slider head to follow the meanderings of a track. However, where servo bursts and a reference pattern are written based upon a desired percentage of a signal (50%) obtained by moving the head incrementally radially out of registry with a previously written adjacent track while reading that track, the undulations and meanderings, both cyclic and not, from the adjacent track are repeated and amended by factors inducing undulations and meanders in the track being written. This process, in essence, converts an undulation in a boundary or edge of an existing signal track sweeping beneath the flux gap of the slider head into a meander, i.e. a displacement in the centerline of the track being written. Such error is further repeated and compounded when the slider head is moved incrementally and positioned radially relative to the just written track based upon signal read for the purpose of writing the next track. Such error continues to grow and evolve until the undulating boundaries overlap and track centerlines meander across one another.
Embedded servo burst patterns defining meandering tracks having overlapping boundaries or having crossing centerlines are not acceptable. Accordingly, bootstrapping servo writing procedures described in the prior art using the actuators servo controllers and slider heads of HDAs have heretofore not been deemed an acceptable method for writing embedded servo.
Classical Adaptive Feedforward Cancellation and Repetitive Control (AFC/RC) techniques such as those described by Sidman (supra) have been suggested for positioning magnetic read write heads in disk drive systems for correcting or compensating for poorly written servo tracks. AFC/RC techniques can also correct for periodic runout and other anomalies in such disk drives. [See paper by M.Tomizuka, Tsu-Chin Tsao & Kok-Kai Chow entitled "Discrete-Time Domain Analyses and Synthesis of Repetitive Controllers" (1987) & paper by Kok-Kai Chow & M.Tomizuka entitled "Digital Control of Repetitive Error in Disk Drive Systems" (1988). Such corrections are required because, in most cases, the conventionally written servo tracks or embedded servo samples have both inherent errors and systemic error. [Systemic error refers to the position error in writing servo samples arising from mis-correlation of exterior "absolute" standards to the inherent internal coordinate system of the HDA.] In fact holding a writing slider head "absolutely" stationary while writing servo bursts to a spinning disk of an HDA when small, low frequency, periodic and repeatable forces are acting on the HDA results in a final drive that will always require AFC/RC in order to function optimally.
Applying classical AFC/RC techniques to servo on a previous track per the teachings of Oliver et al, and Janz (supra) while writing the next can alleviate, to a degree, the tendency for the track center lines to meander or become more and more "out of round" and distorted as a servo pattern is written over many iterations or disk revolutions. However, classical AFC/RC techniques, have drawbacks.
In particular, to function well above the Nyquist limit for frequencies inherently existing in disk drive systems, high sample rates are required. High data processing rates, filtering, fast fourier transform (FFT) operators, and estimaters are also necessary. Meeting these requirements demands large memory and very high speed data signal processors. In addition, non-sinusoidal repetitive errors are impossible to predictably model from drive to drive, and once modeled, difficult to compensate for. However, the biggest drawback is that for classical AFC/RC techniques to work effectively, the drive platform must be extremely well modeled. Even very small variances between the predictive model and the actual HDA platform can introduce errors or, worse yet, compound them by reinforcing rather than canceling non-positional information content in the generated position error signals (PES). The result is embedded servo burst patterns defining meandering tracks having overlapping boundaries and/or crossing centerlines.